Scott,
Engine size aside, what limits G loading on the -9? Knowing the fuselage is essentially the same as the -7, is it the longer wing or longer HS?
I ask because I have always been told the -9 is not aerobatic but there has been nothing more than "don't do it" from Van's.
Actually, the 7 fuselage is the same as the 9... The RV-9 was introduced about 1.5 years before the RV-7
As is discussed here on a regular basis, the proper design of an aircraft is doing it as a whole package, and balance a large list of compromises.
The RV-9 began life with the intent that it would be designed to meet "normal" category certification requirements. In simple terms.... a non aerobatic airplane, designed with limit loads (the load limit a pilot is supposed to avoid exceeding) of 3.8 G's. So the wings and tail were engineered to be able to tolerate 3.8 G's at the selected max. gross weight of 1750 lbs.
The different assemblies were then statistically load tested to
those weights, and confirmed to pass a number of different tests as prescribed by FAR 23.
Beyond that, if anyone is interested in a higher level of detail regarding why the recommended HP limit for the RV-9 is 160 HP, I suggest they read the article previously mentioned titled "ALL THE PRETTY HORSES". You can find it
HERE (it is teh last 2 pages of the document).
If someone takes the time to read it carefully and understand each part before moving on, at the end you should have a good understanding of where the limitations on the RV-9 come from.
In a nut shell.... a properly established Vc speed is one that when an airplane is flying at or below that speed and it encounters a 50 FPS vertical gust (a certification standard based on turb. that can be encountered), it will not induce a load above the max load limit (in the case of the RV-9, 3.8 G's).
The Vc speed is the top of the green arc. An RV-9 with 180+ HP is capable of flying at indicated airspeeds well into the yellow arc range, in level flight. So, the issue has nothing to do with exceeding Vne in a decent, etc.
Ok, but wait a minute, the other 2 seat RV's are able to do the exact same thing when using bigger engines.
True, but there is one key difference, the rest of the 2 seat RV's (the RV-12 doesn't count in this discussion... it is an entirely different situation) are approved for aerobatics. So their structures are designed to take a 6 g's limit, instead of 3.8 (at a reduced aerobatic gross weight, but it still works out to a higher load capability), so there is a broader margin in regards to gust loading.
If we looked at a V-N diagram for the RV-7 to compare, we would see that the gust loading speed limit would graph out higher than it does for the RV-9, and that is why the Vc speed for the RV-7 and 8 is spec'ed at 193 MPH (Relatively close to what the average top end indicated speed in cruise is).
If people want to really understand some of the details regarding what established the limits on the aircraft they fly, read the articles and if needed, ask questions. For the case of the max. HP debate on the RV-9, the last article in the document is particularly relevant.